1. Field of the Invention
The present invention relates to a polyester composition which have improved resistant to hydrolytic degradation and which, when melted, gives little gas emission and undergoes little viscosity change.
2. Description of the Related Art
Polyesters, especially polybutylene terephthalate (hereinunder referred to as PBT) have good moldability, good heat resistance, good mechanical properties and good chemical resistance, and are therefore widely used for various molding materials, for example, for connectors, relays, switches and other parts of automobiles and electric and electronic appliances.
However, PBT is problematic in its hydrolysis resistance. It is considered that the hydrolysis resistance of PBT greatly depends on the carboxyl end group content thereof, and PBT having a reduced carboxyl end group content is desired for realizing increased hydrolysis resistance of itself so that it is usable with no problem even in severe environments, for example, in high-temperature high-humidity atmospheres.
Various methods for the purpose have been investigated. For example, JP-B 27911/1969 discloses a method of adding a phenyl glycidyl ether compound to a polyester; JP-A 87452/1982 discloses a method of adding a monoglycidyl ester compound thereto; and JP-A 52344/1983 discloses a method of adding a glycidyl ester compound and a glycidyl ether compound thereto. However, these methods are all still problematic in that the hydrolysis resistance of the resulting polyesters is not so good and the viscosity in their melts is increased. U.S. Pat. No. 4,229,553, JP-B 47804/1988, JP-A 287657/1991, U.S. Pat. No. 5,026,790, JP-A 287419/1994, JP-A 222279/1993 (U.S. Pat. No. 5,596,049), JP-B 47685/1995 (U.S. Pat. No. 5,300,546) and U.S. Pat. No. 5,731,390 disclose a method of adding an epoxy compound to a polyester and further adding thereto an additive that serves as a catalyst. However, when an epoxy compound is merely combined with a specific catalyst for the additive to a polyester, as in U.S. Pat. No. 4,229,553, JP-B 47804/1988, JP-A 287657/1991, U.S. Pat. No. 5,026,790 and JP-A 287419/1994, it could not still produce satisfactory results. When a single, specific epoxy compound is, combined with a catalyst, added to a polyester, as in JP-A 222279/1993 (U.S. Pat. No. 5,596,049), JP-B 47685/1995 (U.S. Pat. No. 5,300,546) and U.S. Pat. No. 5,731,390, the carboxyl end group content of the resulting polyesters decreases and the hydrolysis resistance thereof therefore increases, but the results are not still satisfactory. In addition, the polyesters disclosed involve another problem in that they give much gas emission when processed or used, and they bleed out when their moldings are hydrolyzed. At present, no one has achieved satisfactory methods for improving polyester.